Plants exhibit remarkable developmental plasticity in response to changing environments. This post-embryonic reorganization requires transcriptional reprogramming at the cell-specific level to initiate new organs to explore the soil for nutrients (see, e.g., Himanen et al., 2004, Proc Natl Acad Sci USA 101, 5146-51). Previous studies have shown distinct differences in the transcriptomes of Arabidopsis thaliana root cells in steady state culture conditions (Birnbaum et al., 2003, Science 302, 1956-60). However, little is known about the extent to which plants modulate gene expression at the cell specific level in response to changing nutrient conditions.
Nitrate is a key required nutrient for the synthesis of amino acids, nucleotides and vitamins and is commonly considered to be the most limiting for normal plant growth (Vitousek et al., 1991, Biogeochemistry 13:87-115). Nitrogenous fertilizer is usually supplied as ammonium nitrate, potassium nitrate, or urea. Plants are keenly sensitive to nitrogen levels in the soil and, atypically of animal development, adopt their body plan to cope with their environment (Lopez-Bucio et al., 2003, Curr Opin Plant Biol 6, 280-7); Malamy et al., 2005, Plant Cell Environ 28, 67-77); Walch-Liu et al., 2006, Ann Bot (Lond) 97, 875-81). For example, mutants in several general nitrogen (N)-assimilation genes affect root architecture (Little et al., 2005, Proc Natl Acad Sci USA 102, 13693-8; Remans et al., 2006, Proc Natl Acad Sci USA 103, 19206-11). Transduction of this nitrogen signal is linked to a massive and concerted gene expression response in the root (Gutierrez et al., 2007, Genome Biol 8, R7; Wang et al., 2003, Plant Physiol 132, 556-67).
Plant development is partially dependent on the plant's response to a variety of environmental signals. For example, the development of root systems is, in part, a response to the availability and distribution of moisture and nutrients within the soil.
In particular, lateral root development in Arabidopsis in response to nitrate is characterized by two distinct pathways. First, an increased rate of lateral root elongation is a localized, direct response to the presence of nitrate in the root zone. (Zhang et al., 1999, Proc Natl Acad Sci 96:6529-6534; Zhang and Forde, 2000, J of Exp. Bot. 51(342):51-59). In this aspect the nitrate ion appears to function as a signal rather than as a nutrient. (Zhang and Forde, 1998, Science 279:407-409). Second, accumulation of high concentrations of nitrate and other nitrogen compounds in the shoot is correlated with a inhibition of root growth through a systemic effect on lateral root meristem activation. (Zhang et al., 1999, supra).
Lateral root primordia formed on roots are the sites for lateral root emergence. Nitrogen treatments of wild-type plants affects (e.g., represses) the emergence of lateral roots. In wild-type plants, a large proportion of root primordia emerge into lateral roots only in nitrogen-poor conditions.
However, it would be advantageous to produce plants that would continue to increase lateral root growth, even in conditions of high nitrogen content in the environment. By increasing lateral root growth or emergence, as well as, for example, enhanced surface area of roots, and/or increased root mass, such plants would be able to assimilate more nitrogen and uptake other essential growth nutrients from the environment (e.g., soil or water) that would otherwise be taken up at much lower rate. Thus, a need remains for plants whose lateral root growth is insensitive to nitrogen content in its environment.